Thermoptic material exhibit changes in physical properties such as electrical conductivity, absorption, reflectivity and refractive index as the result of state changes between the semiconductor and metal states. Thermoptic materials, such as vanadium and titanium oxides, have relatively low infrared absorption in the semiconductor state and high absorption and high reflectance in the metal state.
Thermoptic materials which exhibit metal-semiconductor phase transitions have a reflectance which is temperature sensitive. In thin films of thermoptic materials, there is a transition temperature below which the film behaves as a semiconductor and above which it behaves as a metal. Above the transition temperature of the thin film, the reflectance of the thermoptic material increases abruptly. It is well known that images can be recorded in thin films of thermoptic material at extremely high speeds by selectively heating a region of the film with a laser or electron beam. As long as the ambient temperature of the film is maintained at a suitable bias temperature inside the hysteresis loop, these images will be stored in the film. Normally, such images are erased by allowing the entire film to cool from its bias temperature down to some temperature well below the hysteresis loop.
If the cooling of the film is allowed to occur by natural radiation, conduction or convection to its surroundings, the speed of erasure is slow and impractical for many applications of the thermoptic films. More direct means have also been employed for cooling these films, such as direct cooling with thermoelectric junctions, as disclosed in U.S. Pat. No. 4,283,113 to Eden. U.S. Pat. No. 4,236,156 to Eden discloses another method for erasing thermoptic film displays by generating a high frequency surface acoustic wave on a piezoelectric substrate on which a thermoptic film is employed. Another technique for erasing the film uses a short burst spray of liquid refrigerant to cool the film below its hysteresis loop in a few milliseconds. However, the prior art techniques for erasing the rebiasing thermoptic films are either not fast enough or are not practical in many applications. For example, in dynamic infrared scene simulation, where a laser or electron beam is used to write scene information on the thermoptic film used as an infrared modulator, cycling requirements may require writing and erasing a scene on an individual thermoptic film at speeds as high as about 9 milliseconds. Some prior art erasing techniques are fast enough to meet this speed requirement but require complex and expensive apparatus and are not suitable for use with electron beam writing which requires that the thermoptic film be in a hard vacuum as in a cathode ray tube (CRT). Therefore, a fast method and simple apparatus, suitable for use in a CRT, are needed for erasing and rebiasing thermoptic thin film modulators.